In recent years, many newspaper publishers have begun to print newspapers using a plurality of color inks. Color printing typically requires multiple stages involving a plurality of printing stations. The paper must remain geometrically aligned with each station. Demand has, therefore, increased for a press that can accurately print multi-colors without geometric errors.
One such error, known as fan-out, is caused by anisotropic expansion of paper during the printing process. Paper comprises pulp fibers having longitudinal and radial axes. Moisture causes the pulp fibers to expand. Each fiber tends to expand less than one percent in its longitudinal direction and twenty to thirty percent in its radial direction. The fibers' longitudinal axes extend generally parallel to the length of the paper sheet so that, when moistened, the paper sheet tends to expand significantly more along its width than its length.
Printing involves dampening the paper with water-based inks. This causes the paper to swell, thereby distorting images and lines printed on the paper. Distortion is most evident when printing a plurality of colors in a staged operation. Images created in a first printing station may not coincide with images created at a second or later print station. This includes, for example, misalignment of color pictures with their black borders and colors offset from one another. Print quality can be poor.
Prior art includes mechanical and electronic attempts to reduce fan-out. U.S. Pat. No. 4,404,906 teaches a bowed roller for applying outward lateral force to a paper web in order to expand the web prior to printing a first color. U.S. Pat. No. 5,419,248 describes a device that compensates for expansion of printing plates after successive passes through a print unit. The device includes a register pin for manually adjusting the position of the plate. U.S. Pat. No. 5,619,921 teaches the introduction of waves in a paper web to cancel any expansion caused by moisture expansion. U.S. Pat. No. 5,383,393 includes a plurality of register adjuster means, a paper drawing means downstream of the printing sections, and a plurality of width adjusting means between printing section. U.S. Pat. No. 4,696,230 describes an air nozzle for producing a bustle, or wave, in the paper web, thereby reducing the effective width of the paper web. U.S. Pat. No. 6,024,504 teaches a mathematical process for correcting geometric errors without mechanical intervention.
Prior art includes complex mechanical or electronic systems. Retrofitting to existing printing presses is difficult or expensive. A need exists for an article that reduces fan-out simply and inexpensively. Preferably, the article could be attached to an existing printing press and an operator could adjust fan-out correction during the printing process.